1. Field of the Invention
This invention relates to a condenser bushing comprising a condenser core which is simple in construction and which can uniformly distribute the electric field in the axial direction of the condenser core.
2. Description of the Prior Art
In general, a condenser bushing is composed of a center conductor and a condenser core wound around the center conductor. The condenser core wound around the center conductor is formed by alternatively winding around the center conductor an insulating layer composed of an insulating paper having a large width or a tape-shaped insulating layer and a metal foil or an electrically conductive layer coated with an electrically conductive or semi-conductive paint. In this case, the metal foils or the like for forming the electrically conductive layer, have usually been inserted between the insulating layers and spaced apart from each other by a given distance corresponding to a thickness of 1 mm to 2 mm of the insulating layers. Since the metal foil is manually inserted between the insulating layers, it is impossible to avoid error due to such manual insertion. If the metal foil is displaced in the case of inserting it between the insulating layers, concentration of the electric field at the edge portions of said metal foil occurs, thereby considerably decreasing the electrical insulating ability of the condenser core. In order to avoid this, after the winding operation of the condenser core has been completed, the position of the metal foil must be inspected by X-ray photography. Such inspection is so expensive and troublesome in operation that it cannot be done practically. Therefore, the reliability of the electrical insulation is decreased. In addition, the condenser core winding machine must be stopped every time the metal foil is inserted between the insulating layers for the purpose of checking the inserting position of the metal foil. As a result, if the number of the metal foils to be inserted increases, the productivity of the condenser core is extremely lowered. The higher the voltage of the condenser bushing the longer the condenser core and the larger the thickness thereof, and as a result, the productivity of the condenser core is further lowered.
Many attempts have been made to decrease the number of the metal foils to be inserted between the insulating layers for the purpose of improving the productivity of the condenser core, but hitherto none had led to fully satisfactory results. That is, if the number of the metal foils is reduced, the space between the metal foils becomes large for the electric field to concentrate at the edge portions of the metal foil. As a result, the insulation in the axial direction of the condenser core becomes low, thereby lowering the electric insulating ability of the condenser bushing as a whole.
Heretofore, it has been the common practice to space the metal foils apart from each other by a distance corresponding to a thickness of 1 mm to 2 mm of the insulating layers. Such arrangement of the metal foils results in a frequent stoppage of the core winding machine, thereby lowering the productivity.
Meanwhile, the condenser cores are becoming ever bigger due to the increase of the insulation voltage. The use of such large type condenser core gives rise to various troubles inclusive of the trouble that the core winding operation becomes difficult or the like.